A color picture tube includes an electron gun for forming and directing three electron beams to a screen of the tube. The screen is located on the inner surface of a faceplate of the tube and is made up of an array of elements of three different color emitting phosphors. An apertured mask, which may be either a shadow mask or a focus mask, is interposed between the gun and the screen to permit each electron beam to strike only the phosphor elements associated with that beam. A shadow mask is a thin sheet of metal, such as steel, that is contoured to somewhat parallel the inner surface of the tube faceplate. A shadow mask may be either domed or tensioned. A focus mask comprises two sets of conductive lines that are perpendicular to each other and separated by an insulator. Different voltages are applied to the two sets to create multipole focusing lenses in each of the mask openings. One type of focus mask is a tensioned focus mask, wherein at least one of the sets of conductive lines is under tension. Generally, in a tensioned focus mask, a vertical set of conductive lines or strands is under tension and a horizontal set of conductive lines or wires overlies the strands.
In assembling a tensioned focus mask, it is required to assemble the wires and strands with a high degree of accuracy to achieve consistent spacing. If the spacing precision can be maintained well enough, new manufacturing methods can be used for making the screens of tubes. For example, if, in general, a mask can be made repetitively precise enough, the matrix and phosphor patterns of a screen could be preprinted without using a particular mask as a photomaster. It is therefore desirable to develop techniques for assembling tensioned focus masks that will provide the precision that is required for the new manufacturing methods.